Noel Mackenzie: Understanding the science of agronomy prior to irrigation and drainage

When it comes to irrigation, what do we need to understand about what goes on in that 2-4mm of turf on our greens?

The loss of water from the plant and playing surfaces (transpiration and evaporation – the combined effects are termed evapotranspiration) continues night and day. The rate is lowest during darkness as the temperature is generally lower and the plant has effectively closed for business for the night meaning that the pores in the leaves, the stomata, are largely closed, thereby reducing water loss from the leaves.

During daytime the stomata are open to allow gaseous exchange; to let carbon dioxide (required for photosynthesis) into the plant and let oxygen (the by-product of photosynthesis) out. While the stomata are open water can also escape and this, especially on hotter days, provides a means of cooling the plant. Only when the grass (I am referring to normal temperate climate grasses here, ie C-3 plants) gets very hot and water loss exceeds uptake does wilting occur and at that time the stomata tend to close up or have reduced opening. The stomata close simply because when the plant has a full complement of water it is held open; the stomatal guard cells swollen and tight like two long water-filled balloons. If the water in the balloons is not at full pressure the hole between them becomes smaller. If the stomata get smaller then less water can escape, and, very importantly, gaseous exchange is severely hindered.

The hindering of gaseous exchange at high temperatures due to the closure of the stomata leads to a build-up of oxygen in the plant. This then increasingly blocks further photosynthesis as the trapped oxygen connects to the key photosynthesis enzyme RUBESCO, blocking the further uptake of carbon dioxide. The net result is that the productivity of the plant is reduced and it will come under stress as a result. Without photosynthesis the plant cannot manufacture glucose and from there on all other metabolic processes are limited, including the manufacture of carbohydrates, protein and, from these, new tissue as roots or shoots that equate to growth of the plant. It is well documented that in mid-summer, when temperatures are high, that rooting depth reduces, as does rate of leaf production.  Therefore, just when a plant needs the most help and best developed rooting system, the plant is actually in physiological decline that limits its access to water and the ability to repair itself from mechanical damage. The plant simply cannot sustain itself and begins to decline – a bad scenario when it is only 2-4mm tall!

Understanding the issues

That is the science in a very small nutshell. So what do we as greenkeepers and turf  managers need to understand about the condition of our golf greens in summer?  There are two issues to understand and the management implications from them can be somewhat cryptic:

1. Water has a very important cooling effect on the plant because water requires a significant amount of energy to evaporate.

2. Shortage of water reduces photosynthesis causing stress to the plant which renders it even more prone to drought through reduced rooting depth and biochemical interference.

In practical terms what does this mean for a practicing greenkeeper/turf manager?  Clearly their surfaces will become stressed more than the normal regime of close mowing and other maintenance necessities demand. As stress increases so it starts to distress the plant; this is the point at which the plant gets closer to a catastrophic failure, a point where its ability to survive is overloaded. When areas of a course die this is what has occurred – the stress on the grass exceeded its ability to survive, resulting in either immediate death (acute) or, more often than not, susceptibility to secondary death causes (chronic), such as wear, further drought, heat or wear stress or a pathogen attack, eg Anthracnose disease in Poa annua, which often attacks in later summer/autumn after significant summer stress.

So what’s a greenkeeper to do? Inaction is usually the recipe for disaster in water and heat stress scenarios, action may need to be swift! Let’s consider the obvious and less obvious as options for taking control:

1. Irrigate – OK I know it’s obvious but there are so many issues to consider:

• Is photosynthesis being limited to reduce the health of the grass?

• Are the greens dry enough to warrant watering for the grass to live?

• Are the greens surfaces hot enough to warrant watering to cool the surface?

• What about wind speed, humidity, day and night temperature, precipitation and overall evapotranspiration rates?

• Does my irrigation system give me an accurate value for evapotranspiration rates to allow me to calculate the right replacement value?

• How much should I water?

• How should I water?

• When should I water?

• How should I spread the irrigation intervals/pattern? 3×2 minutes per head, or 1×6 minutes?

• Does my desire to influence playing surface characteristics for the club/owner/members figure in the equation?

• What are my agronomic objectives for the course? How do I balance this against keeping my job?!

2. Management – it’s easier to turn on a tap than to bathe in the whole management mix and figure out the bigger picture. Cultural measures that require analysis are:

• Mowing – heights, frequencies, blade sharpness, patterns, e. not making the perimeter greens cut on a daily basis.

• Mechanical operations – Verti-cutting, grooming, brushing and scarifying treatments – consider depth of operation, frequency, number of passes, blade type and thickness.

• Aeration, type, frequency, tine diameter, depth of operation.

• Magic Potions – are there any miracle products that can help reduce the stress on the grass or off-set the pressure it is under?  These might include:

• seaweed meals/supplements

• changes in topdressing or using topdressing amendments, such as ceramic particles, eg profile, diatomaceous particles, Axis, Humic additives (a whole host are available), moisture holding compounds, eg Zeba, Go Green granules

• high tech fertilisers, eg, sugar based feeds and amino acid supplements, that reduce dependence on providing the plant with raw materials and getting it to make its own dinner but instead providing the plant with the equivalent of a TV dinner!

• plant hormone sprays that block Gibberellins, Abscisic Acid, etc, eg, growth retardants, PrimoMaxx, etc

• consideration of the situation as a whole and questioning how your course got there, whether it was avoidable and whether there is something to be done for next season?

3. PR! This is a very important element in the management of the club, rather than the greens and course.  All the above techniques are worth nothing if the club is not ‘on-side’.  If the course is under stress and changes must be made that may in the short term be detrimental to playing conditions for the long-term good of the course then it is essential that the committee or proprietor is informed and agree to support the measures put in place.

Many turf management changes can be put in place without adversely affecting the playing surface, indeed many may make a dramatic improvement in the short term.  Changes to cutting regimes, mechanical treatments, aeration and especially irrigation are more likely to cause difficulties unless handled very well.  How does changing the irrigation regime impact on the club’s overall agronomic objectives for the golf course?